Roots of the Second Green Revolution

Abstract

The Green Revolution boosted crop yields in developing nations by introducing dwarf genotypes of wheat and rice capable of responding to fertilisation without lodging. We now need a second Green Revolution, to improve the yield of crops grown in infertile soils by farmers with little access to fertiliser, who represent the majority of third-world farmers. Just as the Green Revolution was based on crops responsive to high soil fertility, the second Green Revolution will be based on crops tolerant of low soil fertility. Substantial genetic variation in the productivity of crops in infertile soil has been known for over a century. In recent years we have developed a better understanding of the traits responsible for this variation. Root architecture is critically important by determining soil exploration and therefore nutrient acquisition. Architectural traits under genetic control include basal-root gravitropism, adventitious-root formation and lateral branching. Architectural traits that enhance topsoil foraging are important for acquisition of phosphorus from infertile soils. Genetic variation in the length and density of root hairs is important for the acquisition of immobile nutrients such as phosphorus and potassium. Genetic variation in root cortical aerenchyma formation and secondary development (‘root etiolation’) are important in reducing the metabolic costs of root growth and soil exploration. Genetic variation in rhizosphere modification through the efflux of protons, organic acids and enzymes is important for the mobilisation of nutrients such as phosphorus and transition metals, and the avoidance of aluminum toxicity. Manipulation of ion transporters may be useful for improving the acquisition of nitrate and for enhancing salt tolerance. With the noteworthy exceptions of rhizosphere modification and ion transporters, most of these traits are under complex genetic control. Genetic variation in these traits is associated with substantial yield gains in low-fertility soils, as illustrated by the case of phosphorus efficiency in bean and soybean. In breeding crops for low-fertility soils, selection for specific root traits through direct phenotypic evaluation or molecular markers is likely to be more productive than conventional field screening. Crop genotypes with greater yield in infertile soils will substantially improve the productivity and sustainability of low-input agroecosystems, and in high-input agroecosystems will reduce the environmental impacts of intensive fertilisation. Although the development of crops with reduced fertiliser requirements has been successful in the few cases it has been attempted, the global scientific effort devoted to this enterprise is small, especially considering the magnitude of the humanitarian, environmental and economic benefits being forgone. Population growth, ongoing soil degradation and increasing costs of chemical fertiliser will make the second Green Revolution a priority for plant biology in the 21st century.

Nutritionally Enhanced Staple Food Crops

“…A lot of research has been conducted on the role of root exudates on the acquisition, translocation, and utilization of phosphorus (P) by diverse crop species (Lynch 2007). However, there is a paucity of information on the role of roots and root exudates on the acquisition of Fe and Zn by crops.…”

“…The limited literature available indicates that root exudates are indeed important for the acquisition of Fe and Zn, especially by crops such as chickpea grown on calcareous and alkaline pH soils in which these nutrients are poorly available due to high pH. Moreover, genetic variability exists for Fe and Zn acquisition in chickpea on alkaline pH soils (Ali et al 2002), which is associated with the mobilization of Fe and Zn in the rhizosphere via protons, organic acids, and phenolics in dicots and via phytosiderophores (PS) such as nicotianamine in cereals (Rengel 2002;Welch and Graham 2002;Lynch 2007).…”

“…Equally important are the differences in root exudation among genotypes that differ in tolerance to Fe and Zn deficiency. Genotypic differences in nutrient acquisition-an important determinant of nutrient use efficiency-are associated with root size and morphology, root physiology, increased root (adventitious) production, soil volume explored by roots, and the availability of Fe and Zn in the growing medium (because Fe and Zn deficiency in the soil induces exudation) (Rengel 2002;Lynch 2007;Widodo et al 2010). Further, root surface area can be enhanced through mycorrhizal associations, while the root processes can affect rhizosphere pH and redox potential Garrido et al 2006).…”

NO signaling is a key component of the root growth response to nitrate inZea maysL.

“…11 Among the nutrients present in soil, nitrate (NO 3 − ) may vary by an order of magnitude within centimeters or over the course of a day. 12 The effects of NO 3 − on the root system are complex and depend on several factors, such as the concentration available to the plant, the endogenous nitrogen status and the sensitivity of the species. 10,13,14 A considerable part of the studies aimed to unravel the mechanisms controlling RSA growth and development in response to nitrate have been focused on lateral roots (LR), 8,13,[15][16][17][18][19][20] while the nitrate-regulation of the primary root growth is still unclear.…”

“…1 Despite the vital importance of roots, the difficulty in accessing intact root systems for analysis, particularly under field conditions, have slowed down the breeding programs for plant's adaptation to environmental restrictions. 2,3 The capacity of plants to take up nutrients and water is mainly determined by changes in the architecture of the root system. 1 Three major processes affect the overall architecture of the root system: the rate of cell division, the rate of cell differentiation, and the extent of expansion and elongation of cells.…”